Add perfect grid sizing functionality.

This is an experimental version to make a perfect grid (no gaps). It's
hard coded and not suitable for generic use yet.

For instance, it was added a 'sizes' option that is used to randomily
pick a size for each element on grid. The plugin might disrespect these
sizes for some elements in order to get a perfect grid.
Bugfix: zero elements should not cause error.
Add option to specify first elements sizes.